1. Field of the Invention
The present invention relates to circuit boards having electronic components coupled thereto, and more particularly to electromagnetic interference shielding. Still more particularly, the present invention relates to an electromagnetic interference shield and gap filler for a circuit board.
2. Description of the Prior Art
One of the issues facing manufacturers of electronic devices, such as computers, is electromagnetic interference (EMI). EMI interferes with other electronic circuits and devices. One source of EMI is a circuit board having electronic components coupled thereto. For example, a circuit board having a microprocessor and other electronic components emits EMI.
EMI emissions can be aggravated by a heat sink coupled to some or all of the electronic components on the circuit board. As known in the art, a heat sink conducts heat away from electronic components that require cooling during operation. But a heat sink can also act as an antenna, which intensifies the EMI emissions from the board.
Another problem with a heat sink is that it can damage some of the electronic components. Typically electronic components on a circuit board are coupled to the board such that the top surfaces of the components are at differing elevations from the surface of the circuit board. Consequently, uneven gaps are formed between components and an overlying heat sink. When the heat sink is then thermally coupled to at least some of the components on the board, the heat sink can rock as a result of the gaps. This rocking can damage the corners and edges of the components.
U.S. Pat. No. 5,880,930 by Wheaton discloses an enclosure that functions as both a heat sink and an EMI shield. The enclosure consists of a top housing and a bottom housing made from thermally conductive material such as aluminum. A circuit board having a microprocessor and other electronic components attached to it is housed within the top and bottom housings. The top and bottom housings are held together by four screws. The four screws also connect the enclosure to the logic board. The enclosure reduces EMI emissions while acting as a heat sink as well.
The problem with an enclosure-type EMI shield and heat sink is that it can be expensive to manufacture and makes manufacturing the logic board more complex. Additionally, it makes replacing the microprocessor or other electronic components on the board more difficult, since the EMI shield must be disassembled in order to access the electronic components.
Another technique used to reduce EMI from a circuit board is to add extra ground layers in the board. This allows the components that emit EMI to be better grounded, thereby reducing the EMI emissions from the board. But adding ground layers to a circuit board greatly increases the cost of the circuit board. It also makes manufacturing the circuit board more complex. Moreover, adding additional ground layers in a circuit board may not substantially reduce the EMI emissions. This is because it can be difficult to get a ground layer to the location it is needed most in order to reduce EMI.